1. Field of the Invention
The present invention relates to techniques for IC design, more particularly to a compensation circuit in a feedback loop.
2. Description of Related Art
Miller compensation is widely used for various loop compensation. FIG. 1 is a circuit diagram 100 showing a normal Miller compensation circuit used in a prior art circuit. Miller compensation circuit comprises a transconductance amplifier gm1, a resistor R1 and a capacitor C. An inverse input of the transconductance amplifier gm1 is coupled to a feedback voltage Vf, a non-inverse input of the transconductance amplifier gm1 is coupled to a reference voltage Vr, and the transconductance amplifier gm1 outputs an error voltage Ve via an output thereof. The resistor R1 is connected with the capacitor C in series. One terminal of the resistor R1 is coupled to the output of the transconductance amplifier gm1, and one terminal of the capacitor C is coupled to the inverse input of the transconductance amplifier gm1. The transfer function of the Miller compensation circuit shown in FIG. 1 is:
                              Ve          Vf                =                              -                                                            gm                  ⁢                                                                          ⁢                  1                                +                                  s                  ·                                      C                    ⁡                                          (                                                                        R                          ⁢                                                                                                          ⁢                                                      1                            ·                            gm                                                    ⁢                                                                                                          ⁢                          1                                                -                        1                                            )                                                                                                  s                ·                C                                              ≈                      -                                                            gm                  ⁢                                                                          ⁢                  1                  ⁢                                      (                                          1                      +                                                                        s                          ·                          C                          ·                          R                                                ⁢                                                                                                  ⁢                        1                                                              )                                                                    s                  ·                  C                                            .                                                          (        1        )            where “·” means multiplication. If gm1·R1>>1, the above formula (1) is set up approximately. In the formula, gm1 is a transconductance value of the transconductance amplifier gm1, s is a parameter relative to angular frequency, C is a capacitance value of the capacitor C, and R1 is a resistance value of the resistor R1. Hence, the Miller compensation show in FIG. 1 forms a left half-plane zero the frequency of which equals to
      1          2      ⁢              π        ·        R            ⁢                          ⁢              1        ·        C              .
In many cases, the Miller compensation is desired to form a zero with lower frequency, so the capacitor C with larger capacitance and the resistor R1 with larger resistance are required. For example, it generally requires a 300K ohm resistor of and a 150 pF capacitor to form a compensation zero in some boost power converters. For typical 0.5 micron standard COMS process, the capacitance value of a MOS capacitor with 20 micron length and 20 micron width is about 1 pF. So, the 150 pF capacitor requires 150 number of 20 micron×20 micron areas. The chip area is greatly increased for obtaining the larger capacitance, even if overall design of system will be affected. Additionally, the compensation effect is not satisfactory if a capacitor with lower capacitance is used.
Thus, improved techniques for improved compensation circuit are desired to overcome the above disadvantages.